Star Trek often deals with troublesome subjects that other movies or TV series gloss over. Do you remember the superbeing who committed genocide in STNG's "The Survivors"? The problem with Data's human-like sentience in "Measure of A Man"? The difficulties with eternal life in the Q Continuum (e.g. "Death Wish"?).
The big ideas in Generations are time (friend or foe) and the actual existence of a timeless "Heaven"(i.e The Nexus). These are along the lines of Star Trek's basic theme of "going where no one has gone".
The other themes in the movie are dealing with the loss of family members, realizing one's dreams and making a difference.
Perhaps, the complexity of the story and its subplots was too difficult for some to understand.
Personally, I enjoyed the multi-layered story and its many moving moments like the death of Kirk (twice), Picard seeing his dreams come true in the Nexus and an emotional Data finding his cat alive on board the destroyed Enterprise.
The Nexus obsessed villain, Sorrin, has the most memorable quote in the film: "Time is the fire that we all live in". Four stars.